This is a fallacious argument. You are conflating damage taken over the course of a fight with the damage taken during a very specific portion of the fight. Hallowed Ground is valuable not because of it's overall damage reduction, but its ability to completely ignore specific attacks or dangerous portions of the fight. It allows healers to reposition or heal the raid or top you off. Parry does none of these things; it merely provides a chance for you to take reduced damage from an attack, which may or may not fire off, meaning it is inconsistent. Parry's real value is hopefully allowing your healers to rely more on their smaller, more efficient heals by removing their tendency to freak out when you take a big damage spike via reducing the occurance of those spikes. It CAN save your life, no doubt - but if the only reason you lived through an attack was because you parried it, it's no different from the only reason you living through an attack being the boss rolled low on damage - it's luck, and you should not count on it.
This is exactly the WRONG situation. Parry is most valuable in situations where you have lots of smaller hits incoming so that bad luck is less likely. Think about it this way: you have 50% parry (whatever number that requires) and 50% block, both at 25% DR. The boss has a two hit attack coming at you that does a combined 110% of your remaining health (after any CDs/etc you put up); if both connect you die, but even one parry/block saves you. You die 25% of the time. If that attack is instead 10 hits, say, from 10 smaller mobs instead of a large boss, then you will die less than 2% of the time. Parry is also good in any situation where a single hit will barely kill you, as it provides a chance to live where any other stat might not. That said, the best solution in those situations is usually get a heal.
That's not to say parry can't save your bacon - we've all had times where the tank drops to 50 hp and said, whew, thank god I blocked/parried THAT. I'm not necessarily saying parry is a bad stat; I'm saying it's not good for the reasons you are saying it is. The reason you can solo tank T6 isn't because parry is saving you - if it were, you'd be reliant entirely on luck to survive, and that's terrible tanking strategy. Rather, the reason you are solo tanking T6 is because you and your healers are skilled enough and geared enough in NON-RANDOM stats so that they can consistently keep you alive through your cooldowns and the increasing stacks of the boss.